Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 131125

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
625 AM CDT Fri Oct 13 2017

341 AM CDT

Through tonight...

The low cloud cover that has been dominate across the area over
the past couple of days has been breaking up a bit overnight
across portions of the area. In these areas, temperatures have
dropped into the upper 40s to lower 50s allowing some fog to
develop early this morning. While this fog is likely to dissipate
early this morning, it does appear that cloud cover will redevelop
over most of the area today, and this should result in another
mostly cloudy day across the region, especially by late morning
and into the afternoon. These clouds are likely to hold
temperatures down into the upper 60s north, to lower 70s south

A cold front, emanating from low pressure up over southern
portions of Hudson Bay, will approach northern Illinois later
this afternoon and evening, before briefly stalling out over
northern Illinois tonight. This front will act as the focus for
showers and thunderstorms. While most of the area will remain
precipitation free through the day, shower and thunderstorm
chances will be on the increase into early this evening over north
central Illinois. Additional showers and thunderstorms are likely
to develop over northern Illinois overnight as moist warm air
advection (isentropic upglide) starts to ramp up overnight in
response to a strengthening southwesterly low-level jet tied to
developing low pressure over the Central High Plains. While severe
weather is not expected with this activity, these showers and
storms will be capable of producing some locally heavy rainfall.



341 AM CDT

Saturday through Thursday...

The main concerns will focus on the potential for heavy rainfall
and possibly strong showers and thunderstorms on Saturday.

A potent upper trough is forecast to dig over the Rockies and drive
the development of surface low pressure over the Central
Plains by early Saturday morning. The low will ultimately shift
northeastward across eastern Iowa and into southern Wisconsin,
while deepening to near 1000 MB, by Saturday evening. An attendant
cold front will then be driven eastward over the area Saturday

Periods of showers and some thunderstorms are likely to continue
at times on Saturday as strong southwest flow aloft continues to
drive disturbances over the area. This will be especially true
for much of far northern Illinois. Locally heavy rainfall will
continue to be possible with this activity as precipitable water
(PWATs) values are likely to be north of 1.5".

As the surface low begins to shift across eastern Iowa later in
the day, expect the surface boundary over the area to begin
shifting northward into Wisconsin, before the cold front begins to
shift eastward over the area into Saturday night. Strong to
severe showers and thunderstorms will be possible along this cold
front, especially to our west-southwest, Saturday afternoon and
evening as the wind field really ramps up in response the
deepening low. This should set up very impressive deep and low-
level shear profiles over much of the warm sector into Saturday
night. As a result, the possibility for strong to severe storms
shifting into portions of northern Illinois Saturday night cannot
be ruled out at this time. The main lacking ingredient will be
instability. This is a result of extensive cloud cover and periods
of showers and storms expected over the area ahead of the front,
as well as the poor diurnal timing of the frontal passage itself.

However, in spite of this, the high shear-low instability
environment may still support a line of strong showers and storms
capable of producing strong damaging winds into northern Illinois
Saturday night. This is especially a concern given that the winds
are forecast to be around 50 KT within only about 2,000 feet off
the surface, and would likely not take much to mix them towards
the surface. On top of this severe concern, very heavy rainfall
and possible hydro issues will also continue to be likely with
these showers/storms with PWATs possibly near record highs for
this time of year (above 1.8"). Given the higher confidence in the
heavy rain threat, and the recent heavy rain, we will be issuing
a ESF for portions of northern Illinois.

Aside from being windy and cool on Sunday, the weather should
begin to quiet down following the cold frontal passage. It appears
that a long period of mild and dry weather will set up over the
area for much of next week as high pressure dominates across much
of the eastern half of the country.



For the 12Z TAFs...

There is still quite a bit of MVFR cloud in the area with some
lowering of cloud bases. There are also some pockets of clearing.
In a moist atmosphere these pockets of this clearing area quickly
fill back in with either lower visibility, sometimes briefly
dense, but the common cloud deck filling back in is around 1500

The question for later today becomes how much clearing occurs and
the timing. Forecast soundings show the MVFR cloud layer thinning
to the point where we do feel ORD/MDW will see some lifting.
Higher confidence exists for clouds to go VFR at MDW, but ORD
stands a decent chance too. SW winds will back to SE late this
afternoon and evening ahead of low pressure. Also expect showers
to fill in from west to east, as early as late afternoon across
RFD then spreading east and south. Confidence is high in precip
filling in overnight, but it could occur a bit earlier. Expect a
return to MVFR conditions in the rain, which may be heavy at
times, along with the possibility of thunder, mainly in the 8z-
12z time frame.

A frontal boundary will stall across the area on Saturday, which
will lead to additional rounds of showers and thunderstorms and
lower ceilings.



341 AM CDT

After a quiet wind pattern today, active and changing
conditions will bring highly varying winds for the weekend,
including a concern for gale force winds later Saturday night into
Sunday. High pressure will shift east tonight, while a stronger low
passes well to the north. This low will send a cold front southward
across the lake tonight not quite reaching the southern tip by
daybreak Saturday. This front will shift winds to NNE for areas
behind the front. Another modest low will take shape across Iowa and
cruise northeastward. This low will lift the front back north and
lead to near or possibly to gale force SSW winds Saturday evening
ahead of the low. The low will strengthen further as it continues
into Quebec. With the first seasonally strong cold front behind this
low, expect a period of gale force northwesterly winds late Saturday
night into Sunday. Another low will pass north of the lake early
next week, and this will shift winds back to the southwest.






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